The use of semipermeable membranes for reverse osmosis or ultrafiltration processes is well known. For example, in a reverse osmosis process, high pressure saline water may be placed in contact with a semipermeable membrane which is permeable to water but relatively impermeable to salt. Concentrated brine and relatively pure water are separated thereby; the water may then be utilized for personal use such as drinking, cooking, etc.
More recently certain membranes have been utilized for the separation of various gases. The separation of a gas mixture utilizing a membrane is effected by passing a feed stream of the gas across the surface of the membrane. Inasmuch as the feed stream is at an elevated pressure relative to the effluent stream, the more permeable component of the mixture will pass through the membrane more rapidly than the less permeable component(s), affording a permeate stream (that passing through the membrane) enriched in the more permeable component and a residue stream enriched in the less permeable component(s) of the feed stream.
This ability to separate gases from a mixture stream finds many applications in commercial uses. For example, gas separation systems may be used for oxygen enrichment of air, for improved combustion efficiencies and conservation of energy resources. Likewise, nitrogen enrichment of air may be applicable where inert atmospheres are required. Other applications for oxygen enriched gases may be improving selectivity and efficiency of chemical and metallurgical processes. Similarly, inert atmospheres such as may be provided for by this invention may also be utilized in chemical and metallurgical processes. Some other applications of gas separation include helium recovery from natural gas, hydrogen enrichment in industrial process applications, and scrubbing of acid gases. Specific uses for oxygen enrichment of air are breathing systems for submarines and other underwater stations, improved heart-lung machines, and other lung assist devices. Another specific application of a gas separation system is an aircraft to provide oxygen enrichment for life-support systems and nitrogen enrichment for providing an inert atmosphere for fuel systems. In addition, oxygen enriched air can be used in furnaces for more efficient combustion and in catalytic oxidation of organic compounds, e.g., mercaptans, hydrocarbons, alcohols, aldehydes, etc., to name but a few. Likewise, gas separation systems may be used for environmental benefits, e.g., methane can be separated from carbon dioxide in waste gases for sewage treatment processes and oxygen enriched air can be produced to enhance sewage digestion.
The separation of gases by a selective separation process will provide a product which possesses a different proportion of the gases than was present in the original feed mixture. The membranes which may be utilized to effect such a selective separation must possess the ability to withstand the conditions to which they are subjected during the separation operation and must provide a sufficiently high flux so as to permit the use of these membranes in a commercially attractive process. Therefore, it is necessary to provide membrane composites which exhibit a highly selective separation with regard to various gases as well as providing an economically attractive flux.
Membranes which are composites of a thin polymer film on a porous support have been reported. For example, U.S. Pat. No. 3,892,665 discloses a thin polymer film which is formed on the surface of a liquid, generally water, and is subsequently transferred to the surface of a porous supporting membrane. During the transfer of the thin polymer film, the porous support is maintained in a wetted stage with the liquid. Alternatively, the thin film can be formed on the surface of the porous membrane if the surface of the support is first wet with the transfer liquid. In either case the pores of the support member must be filled with liquid and, therefore, the liquid must be removed from the porous support at a period subsequent to the formation of the film in order to draw the film onto the support. In general, such a polymer film is a monomolecular layer which is formed on the surface of the water wherein the individual film-forming monomer and/or polymer chains are oriented and closely packed. Subsequently, the oriented monomolecular layer or film, which is limited to a thickness in the range of from about 5 to about 25 Angstroms, is transferred to the surface of the porous support membrane. This process may be repeated until multiple monolayers are deposited on the surface of the support, the total film thickness then being from about 10 to about 200 Angstroms. Other than van der Waals' forces, there is no bonding between the aggregate layers and the support, which means that the thin film of the finished membrane is weakly attached to the porous support and said membrane cannot withstand substantial back pressure when in operation. Obviously, this process is tedious and expensive and is not readily amenable to commercial use.
U.S. Pat. No. 3,526,588 discloses a macromolecular fractionation process and describes a porous ultrafiltration membrane which is selective on the basis of pore size. In contradistinction to this, it is essential that a thin film membrane for gas separation be nonporous, so that separation operates by a diffusion-solution mechanism of transport. U.S. Pat. No. 3,767,737 which discloses a method for producing castings of "ultra-thin" polymer membranes is similar in nature to U.S. Pat. No. 3,892,665 in that the thin film of the membrane is formed on the surface of a liquid and transferred to the surface of a porous support membrane. The thin film polymer will thus inherently possess a disadvantage ascribed to the membrane of the former patent in that it cannot withstand substantial back pressure when in operation. In addition, U.S. Pat. No. 2,966,235 discloses a separation of gases by diffusion through silicone rubber which is not composited on a porous support material.
U.S. Pat. No. 4,155,793 involves a continuous method for the preparation of membranes by applying a polymer to a microporous support. However, the method of production described in this patent involves the spreading of a polymer casting solution onto the surface of a liquid substrate. The polymer which is utilized is not soluble in the liquid substrate nor is the solvent which is used compatible with the microporous support. The polymer film which constitutes the membrane is formed on the surface of the liquid and is thereafter applied to the microporous support. U.S. Pat. No. 4,132,824 discloses an ultra-thin film of a polymer composite which comprises a blend of a methylpentene polymer and an organopolysiloxane-polycarbonate interpolymer for a thickness less than about 400 Angstroms in which the interpolymer is present in an amount of up to about 100 parts by weight per 100 parts by weight of the methylpentene polymer. Likewise, U.S. Pat. No. 4,192,824 describes a method for preparing the aforementioned interpolymer by depositing on the surface of a liquid casting substrate a casting solution which comprises a mixture of methylpentene polymer and from 0 to 100 parts by weight of an organopolysiloxane-polycarbonate copolymer. The casting solution spreads over the surface of the liquid casting substrate to form a thin film following which at least a portion of the thin film is removed from the surface of the substrate. Thereafter, the film may be used in contact with a porous support as a gas separation membrane.
Other patents have also described various membranes for effecting a gas separation. In this respect, U.S. Pat. No. 4,230,463 describes a multicomponent membrane in which a material which exhibits selective permeation of at least one gas from a gaseous mixture is in occluding contact with a porous separation membrane. Various types of polymers are cited as being suitable for the porous separation membrane, the preferred polymer being a polysulfone. It is also stated in this patent that the porous separation membrane is preferably at least partially self-supporting and in some instances may be essentially self-supporting. European Patent Application No. 0031725 is drawn to an ultrathin solid membrane process which may be used for gas separation. This membrane is prepared by dissolving an additional polymer derived from at least one monomer selected from ethylenically unsaturated hydrocarbon monomers and conjugated unsaturated hydrocarbon monomers in an organic liquid medium which may, if so desired, contain another organic compound such as an alcohol, ketone, aldehyde, carboxylic acid, etc. The solvent solution is then spread on a liquid support such as water and the desired membrane is formed on the surface thereof.
U.S. Pat. No. 3,335,545 is drawn to a process for gas separation by differential permeation, said process being effected through liquid or quasi-liquid films which behave substantially as polymeric films. Another U.S. Pat. No. 3,951,621, is drawn to a process for separating one or more components of a gaseous mixture utilizing a membrane of cross-linked hydrophilic poly(vinyl alcohol) and a polyamide such as nylon. In addition, the film contains complex-forming metal components which are active in the presence of water. The films are used as a free-standing self-supporting membrane. U.S. Pat. No. 4,248,913 discloses a process for preparing a membrane which comprises a blend of a vinylidene fluoride polymer and a hydrolyzed vinyl acetate polymer which forms a self-supporting film used in ultrafiltration processes. U.S. Pat. No. 4,302,334 also discloses a microporous polymeric membrane based upon the membranes set forth in U.S. Pat. No. 4,248,913. In addition, the membranes may also include copolymers of vinyl acetate with other components such as acrylates, maleates and ethylene.
The membranes described in U.S. Pat. Nos. 3,556,305 and 4,439,217 are superficially, but only superficially, analogous to the membrane of our invention. The former relates to membranes used in ultrafiltration and reverse osmosis where the membrane is a composite of a porous substrate, an adhesive, and a diffusive polymer or gellike film. Among the diffusive polymers was mentioned a mixture of poly(vinyl methyl ether) with a copolymer of vinyl methyl ether and maleic anhydride. U.S. Pat. No. 4,439,217 teaches a permselective layer for gas separation composed of an organic polymer having pivalate groups in the side chains. The pivalate group is taught as being the permselective element, and the patentee teaches that pendant pivalate groups can be incorporated via homopolymerization of vinyl pivalate or copolymerization of the latter with other monomers including vinyl ethers generally and vinyl isobutyl ether specifically. But the patentee stresses that such comonomers are used solely to impart useful physical properties wholly unrelated to permselectivity. Consequently it is fair and accurate to state that the prior art is devoid of any teaching that homopolymers of vinyl alkyl ethers, and especially those of our invention, are useful as permselective elements in membranes for gas separation.
As previously mentioned, the separation of various gases from a mixture thereof may become increasingly important in view of the necessity to conserve energy. A particular application would relate to increasing the thermal efficiency of combustion processes when utilizing fossil fuels in commercial combustion applications. Also, by utilizing a gas separation membrane in coal gasification, it may be possible to provide an oxygen enrichment of air for the production of low and medium British Thermal Unit (BTU) product gases as well as an oxygen enrichment of air for the combustion of these gases. For example, by placing a gas membrane separation system in close proximity to both gas production and gas combustion facilities, it would allow a site-located oxygen enrichment plant to supply both processes without the additional expense of transporting the gas or duplicating enrichment facilities.
The requirements for an efficient oxygen enrichment membrane, for example, include the characteristics of being thermally stable at moderately elevated temperature; the ability to withstand high pressures without physically destroying the membrane; hydrolytic stability to water and/or water vapor; and existence in a physical form which is adaptable for use as a thin film composite membrane in sheet form or as a coating to hollow fine fibers. As will be shown in greater detail, we have now discovered a gas enrichment membrane composite which will possess all of the desirable characteristics enumerated. In particular, we have found that a membrane of a porous backing support having a permselective layer of a homopolymer of certain vinyl alkyl ethers is a quite convenient composite for various gas separations. In particular, homopolymers of such ethers where the alkyl group is branched either at the carbon bonded to the ether oxygen or at the next adjacent carbon atom form an especially useful permselective layer.